Washington (CNN) -- A counterfeit version of the anti-cancer drug Avastin may have been purchased and used in a number of medical facilities in the United States, the U.S. Food and Drug Administration warned Wednesday.

The FDA sent letters to 19 medical practices known to have purchased unapproved cancer medications, according to the agency.

The counterfeit version of Avastin lacks the medicine's key ingredient, bevacizumab.

Roche, the pharmaceutical company that manufactures Avastin, conducted laboratory tests confirming the presence of the counterfeit version, the FDA noted.

"The counterfeit product is not safe or effective and should not be used," Roche member company Genentech said in a written statement. Genentech is responsible for marketing Avastin.

The unapproved cancer medications -- potentially including the counterfeit version of Avastin -- were purchased from a foreign supplier known as Quality Specialty Products, which may also be known as Montana Health Care Solutions, according to the FDA

"It sounds like the key issue with that ... is that the counterfeit vials don't have Genentech labels on them," said Donald Harvey, president of the Hematology and Oncology Pharmacists Association. "Pharmacists have to take a look and really investigate their supply."